Recent technologic advances have made it possible to tailor assays for a wide variety of analytes, especially those molecules exhibiting antigenic characteristics. In general, most assays currently in use tend to use antibodies to bind antigenic substances in a liquid-phase or a solid-phase format. Recently, "one-step" immunoassays have been developed, which are used in the detection of a variety of analytes.
One-step immunochromatographic assays involve the use of colored or other visible particles. For example, a sample is applied to a substrate of absorbent material, and analyte binds to antibody bearing mobile colored latex particles. The particles to which the analyte binds are then themselves bound by immobilized immunoreagent as the sample chromatographically traverses the length of the absorbent material.
Assay sensitivity is defined as the minimum amount of analyte that can be measured with acceptable precision. One area where assay sensitivity is particularly problematic is in the assay of fecal matter to detect the presence of occult blood. A small amount of blood is normally present in human stool samples. Increased amounts of blood in the stool are indicative of pathological bleeding. Currently available assays are not sensitive enough to detect abnormal amounts of fecal occult blood until the level reaches about 1 mg hemoglobin/g feces. Lesser quantities of blood in the stool, however, are indicative of abnormal bleeding. Thus, currently available assays for fecal occult blood are not sensitive enough to detect the presence of abnormally high levels of blood in the stool. However, the sensitivity of any assay for fecal occult blood should not be increased to the point where normal levels of blood in the stool give a positive test result.
There is therefore a need for a simple, accurate, reproducible assay for the detection of analytes such as fecal occult blood, which provides controlled, predetermined assay sensitivity.